The present invention relates to a downhole safety valve for a central circulation completion system. More particularly, the invention relates to a valve for selectively sealing the annular production bore between the inner tubing string and the outer tubing string in a central circulation completion system.
In traditional subsea completion systems, a tubing string is suspended from a tubing hanger which is landed in a wellhead housing or in a christmas tree that is installed over the wellhead housing. The tubing string extends into the hydrocarbon reservoir and defines a production bore through which well fluids may be produced. In addition, the tubing string is usually positioned within a casing string which is typically suspended from the wellhead housing. The casing string also extends into the reservoir, and the annulus between the tubing string and the casing string defines a service bore through which fluids may be circulated, for example during installation of the completion system.
Subsea completion systems normally comprise at least one downhole safety valve, such as a surface controlled subsea safety valve (“SCSSV”), which functions to close off the production bore in the event of an emergency. A typical prior art SCSSV comprises a ball valve which is disposed in a cylindrical housing that is mounted within or between successive sections of the tubing string. The ball valve is held in the open position by hydraulic control pressure but is biased toward the closed position by a spring or the like. Accordingly, if a need to close off the production bore arises, the hydraulic pressure is relieved, thus allowing the valve to close.
In central circulation completion systems, examples of which are described in International Publication Number WO 01/81710 A1, which is hereby incorporated herein by reference, the roles of the production and service bores are largely reversed. Thus, in such systems the inner tubing string defines a service bore which may be used to circulate fluids during installation and workover operations. In addition, the annulus between the inner tubing string and a second, outer tubing string defines a production bore through which well fluids are produced.
As with traditional subsea completion systems, central circulation completion systems must also be provided with means to shut off the production bore in the event of an emergency. However, since most downhole safety valves are designed to be used with conventional completion systems, wherein well fluids are communicated through the inner tubing string, they generally cannot be used with central circulation completion systems.